Myelodysplastic syndromes are clonal hematological diseases with heterogeneous clinical and laboratorial presentations, which result in progressive bone marrow failure and evolve to acute leukemia. Anemia is a common symptom. In elderly patients, it is not attributed to the normal aging process and the cause is identified in most cases. The presence of cytopenias associated with bone marrow dysplastic disorders may also be due to secondary and reversible non clonal disorders. Cytogenetic abnormalities found in a proportion of patients with myelodysplastic syndromes may be helpful in the differential diagnosis and to evaluate the prognosis. Ancillary laboratory tests to show clonality are not usually available. The diagnosis of myelodysplastic syndromes is, therefore, made by exclusion, sometimes helped by the passage of time. Considering the proposed multistep myelodysplastic syndrome pathogenesis, patients at the lowest grade, presenting minimal dysplastic features may be difficult to diagnose. Vitamin B12 and/or folate deficiency, recent exposure to heavy metals and recent cytotoxic or growth factor therapy should be considered absolute exclusion factors precluding the definite diagnosis. Alcohol use, chronic inflammatory states, auto-immune disorders, chronic liver or kidney diseases, hormonal disorders and viral infections including HIV must be ruled out or interpreted with caution. Some diseases of the pluripotential stem cells must also be distinguished from myelodysplastic syndromes. Exclusion of paroxysmal nocturnal hemoglobinuria and aplastic anemia may be difficult in the less common hypocellular myelodysplastic syndromes. Dysplastic abnormalities of the bone marrow, therefore, do not in themselves establish a diagnosis of myelodysplasia and a protocol of exclusion should be carried out.